The evolution of intelligence, and why our brains have shrunk

Scientists have called humans the "uniquely unique species" due in large part to the size of our brains in relation to our bodies and our complex social interactions.

What science has had trouble explaining is why, 100,000 years ago, over a relatively short period in biological evolution, we developed such large brains at a time when the functions made possible by a large brain would seem unnecessary.

A paper [1] published in the Proceedings of the National Academy of Sciences by University of Tennessee professor Sergey Gavrilets looks to Niccolo Machiavelli for the answer.

Gavrilets, a professor of ecology and evolutionary biology and mathematics, developed a mathematical model to test the theory that humans' large brains are a result of the most clever males using increasingly sophisticated strategies to outmaneuver other males in the quest to reproduce as much as possible.

Some evolutionary biologists have theorized about the idea of Machiavellian intelligence - namely, that there is a distinct advantage to the types of trickery and deception espoused by the 14th century Italian noble Niccolo Machiavelli in his pamphlet entitled "The Prince." They theorize that the advantage is seen most strongly in the task of reproduction.

"Mating success is a powerful determinant in genetic selection," said Gavrilets. "This research indicates that the most successful males were those who beat out other males in their quest to reproduce."

A somewhat more modern example of this concept is Genghis Khan. The 12th century warrior, famed for building the largest empire in world history and for his unusually high amount of reproductive activity, left a striking genetic mark on the Eastern world that can still be seen today. Eight percent of all Asian males carry a chromosome which appears to be directly linked to Genghis Khan, said Gavrilets.

Gavrilets admits that his results do not reflect the moral values shown by modern humans, and that they could be seen as controversial.

"The idea of competition for reproductive success as the main driver of our evolutionary leap is not what you would call politically correct," said Gavrilets.

In fact, the results of Gavrilets' mathematical model show that after a burst of growth in brain size resulting from a spike in Machiavellian intelligence, there should be a leveling off of both Machiavellian behavior and brain size, apparently corresponding to our current stage of development.

The model, developed with the help of UT computer science undergraduate Aaron Vose, looks at genetic factors in the increase of brain size as well as society's influence through the spread of "memes." They define a meme as an idea or strategy that can be learned from other members of the group and that can be used to subvert others as people look to gain social power.

Vose and Gavrilets used powerful computer clusters to test their model over thousands of "generations" of humans, and found that the evolution of brain size happened in three stages.

First, there is an extended waiting period during which there is little sign of the evolution to come; second, a "cerebral explosion" during which the learning ability, brain size, number of memes and overall Machiavellian fitness grow very rapidly; and finally, a saturation phase in which these qualities cease to grow and in some cases slightly decline.

Gavrilets notes that his research does not advocate for Machiavellian behavior as a way for humans to further evolve.

"It is important to point out that there's a difference between how you acquire a tool like a large brain, and how you use it once you have it," said Gavrilets.

The "Machiavellian intelligence" hypothesis (or the "social brain" hypothesis) posits that large brains and distinctive cognitive abilities of humans have evolved via intense social competition in which social competitors developed increasingly sophisticated "Machiavellian" strategies as a means to achieve higher social and reproductive success. Here we build a mathematical model aiming to explore this hypothesis. In the model, genes control brains which invent and learn strategies (memes) which are used by males to gain advantage in competition for mates. We show that the dynamics of intelligence has three distinct phases. During the dormant phase only newly invented memes are present in the population. During the cognitive explosion phase the population's meme count and the learning ability, cerebral capacity (controlling the number of different memes that the brain can learn and use), and Machiavellian fitness of individuals increase in a runaway fashion. During the saturation phase natural selection resulting from the costs of having large brains checks further increases in cognitive abilities. Overall, our results suggest that the mechanisms underlying the "Machiavellian intelligence" hypothesis can indeed result in the evolution of significant cognitive abilities on the time scale of 10 to 20 thousand generations. We show that cerebral capacity evolves faster and to a larger degree than learning ability. Our model suggests that there may be a tendency toward a reduction in cognitive abilities (driven by the costs of having a large brain) as the reproductive advantage of having a large brain decreases and the exposure to memes increases in modern societies.